1. Field of the Invention
The invention relates to a method of making ohmic and/or Schottky barrier diode contacts to a semiconductor substrate, more particularly, to a method of making oxide-free contacts and to an apparatus for implementing the method.
2. Description of the Prior Art
The requirements for a material or combination of materials to provide ohmic and Schottky barrier diode contacts to semiconductor substrates are extremely high both from an electrical and chemical standpoint.
Numerous metallurgical systems which are known to semiconductor designers have been proposed and utilized as ohmic and Schottky barrier diode contacts. The single most successful metal used in interconnections of integrated circuits is aluminum or aluminum doped with a small amount of copper or silicon. Aluminum makes good ohmic and mechanical contact to the silicon and to the surrounding insulating layers. It is easy to deposit by standard evaporation or sputtering techniques and can be easily patterned by etching or similar techniques. However, aluminum has a tendancy to interact with silicon, particularly at high temperature processing, causing short circuits between the aluminum and the semiconductor substrate. In addition, aluminum alone forms neither a very high, nor a low, barrier height Schottky barrier contact to silicon.
Another metallurgical system that has been suggested for utilization as ohmic and Schottky barrier contacts to silicon semiconductor substrates is a metal-silicon compound known as a silicide. In this connection silicides such as platinum silicide, palladium silicide, nickel silicide, rhodium silicide, zirconium silicide, hafnium silicide, have been proposed. A prior art reference in this context is IBM Technical Disclosure Bulletin, Vol. 13, No. 3, August 1970, pp. 646-648, C. I. Kirchner and H. N. Yu, "Fabricating a Gate Field-Effect Transistor".
Reference is made to U.S. Pat. No. 3,274,670 which discloses a method of forming low-resistance electrical contacts to semiconductor devices by depositing on a silicon substrate a thin layer of platinum and heat treating to form platinum silicide. Thereafter, a composite layer of platinum, titanium and gold is deposited over the platinum silicide.
Reference is made to U.S. Pat. No. 3,290,127 which discloses forming an active contact layer of palladium silicide as a surface barrier with the semiconductor material. Silver-gold metallization is then formed over the contact layer. According to this patent other suitable metals in place of palladium include nickel, copper, rhodium, platinum, tungsten, molybdenum.
U.S. Pat. No. 3,893,160 discloses a resistive connecting contact for a silicon semiconductor substrate formed from a layer sequence of platinum silicide-titanium-molybdenum-gold.
Referring to the prior art on Schottky barrier diode contacts, it is known, for example, from U.S. Pat. No. 3,906,540 to form on the surface of a silicon semiconductor body a layer of a metal silicide such as platinum silicide, then applying a refractory metal barrier layer such as molybdenum, titanium, tungsten, tantalum, followed by forming an aluminum contact layer. The refractory metal barrier layer prevents intra-diffusion of aluminum and silicon constituents during subsequent heat treatments.
From U.S. Pat. No. 3,995,301 a Schottky barrier structure is known where a metal layer of Al.sub.2 Pt is in contact with a high resistivity semiconductor region. The structure is fabricated by first forming a platinum silicide layer on a silicon substrate and then applying an aluminum layer thereon after which the structure is sintered at a temperature of at least 400.degree. C. for at least one hour.
Thus, the literature is replete with a multitude of metallurgical systems which fulfill one or more functions as metallurgical contacts. One of the most successful of these systems for ohmic contacts is platinum silicide/aluminum and for Schottky barrier contacts is a system which utilizes a barrier layer consisting of chromium, titanium, tungsten or a titanium-tungsten alloy between aluminum and the silicide layer.
The above discussed metals which are known for their suitability for contact metallization are usually applied on carefully cleaned silicon semiconductor subtrates provided with a mask layer having the desired contact openings, typically, by electron beam evaporation or cathode sputtering techniques in a high vacuum at temperatures higher than approximately 350.degree. C. These methods of application in a high vacuum present a number of problems which hitherto could not be solved satisfactorily. For example, in the production of platinum silicide semiconductor contacts by evaporation of platinum in a high vacuum at a temperature of approximately 350.degree. C. and subsequent sintering, forming of platinum silicide in the contact region is impeded by the presence of a thin surface layer of silicon which is rich in oxygen. This oxygen-rich layer, which can be detected by means of Auger-electron spectroscopy, causes a deterioration of the electrical contact properties. Schottky barrier diodes fabricated in this manner exhibit a high contact resistance and a non-linear behavior. Also, during the cleaning of the contacts prior to the evaporation of the aluminum metallization it was even possible to completely remove the platinum silicide due to etching of the underlying oxygen-rich layer. The application of other metals for the contacting of semiconductor substrates revealed similar phenomena.
It is believed that the oxygen-rich layer is a layer of silicon dioxide which grows in the exposed contact regions of the silicon substrate during the above-mentioned 350.degree. C. process step, due to the presence of oxygen and steam residues in the vacuum chamber. Consequently, to improve the contact behavior attempts were made to reduce the oxygen content in the vacuum chamber. One approach is to improve the vacuum in the evaporation by installing a Meissner trap and a cryopump. However, the long cooling and heating cycle of the Meissner trap reduces the throughput.
Another approach is to reduce the substrate temperature during the deposition of the platinum layer. The disadvantage of this method, however, is poor adhesion of the platinum silicide film to the silicon semiconductor substrate resulting in poor electrical contact properties. Attempts were also made to initially deposit the platinum film on cold silicon wafers and as the deposition progressed heat the wafers to a temperature of approximately 350.degree. C. The disadvantage of this method is that it is difficult to control and therefore not suitable for manufacturing purposes.
The cleaning of the substrate surface by means of high frequency cathode sputtering in a vacuum chamber prior to the deposition of a metallic film has been described in the literature (e.g., see the article by R. M. Anderson and T. M. Reith in J. Elec. Chem. Soc., Vol. 122, 1337, 1975). This method although leads to a cleaning of the substrate surface, due to resputtering during cleaning metallic impurities such as iron and nickel are deposited on the surface of the silicon wafers. In addition to forming metallic impurities, the cleaning technique does not fully remove the oxygen.
Reference is also made to German Offenlegungsschrift No. 2,758,576 which concerns a high temperature method for reducing traces of undesired heavy metals such as nickel, copper, iron and gold which entered the doped semiconductor wafer during fabrication of silicon semiconductor devices. In this method one or more doped semiconductor wafers are arranged along the longitudinal axis of a sealed quartz tube and flanked on either side by high purity unprocessed silicon semiconductor wafers. The undesired traces of heavy metals are removed by maintaining in the quartz tube an inert gas atmosphere at a pressure of approximately 1.times.10.sup.-6 torr and temperature of 1100.degree. C. for a period of at least 10-60 minutes.